Spacecraft, satellites, and other space systems typically employ many sensitive and expensive components for performing various space functions, such as information gathering and transmission and optical imaging. When in space, a spacecraft is exposed to vast temperature extremes of heat and cold. Thermal radiation from the sun may heat a spacecraft and the spacecraft""s components to very high temperature levels which may damage the spacecraft. Visible light is the main source of thermal energy in the space environment. Therefore, it has heretofore been known in the art to provide passive thermal control in the form of second surface mirrors, special paints, multi-layered thermal blankets, or sunshields over sensitive spacecraft components to impede, reflect, and/or emit harmful solar radiation to prevent spacecraft components from being excessively heated.
Thermal control of a spacecraft is typically provided by radiating heat from external spacecraft surfaces. The solar heating and cooling of the spacecraft is determined by the quantity of energy absorbed and the quantity of energy emitted by the spacecraft. The bulk of the energy absorbed by the spacecraft when the spacecraft is in space is from the peak of the solar radiation spectrum at wavelengths between 400 to 800 nm. The bulk of the energy emitted is from the infrared spectrum at wavelengths greater than 1000 nm. The applicable equation of solar heating is provided by:
xcex1A1E0=xcex5A2ST4, orxe2x80x83xe2x80x83(1) 
T=[(xcex1/xcex5)(A1/A2)(E0/S)]1/4xe2x80x83xe2x80x83(2) 
where xcex1 is the solar absorptance of the spacecraft surface, A1 is the solar absorbing area, E0 is the solar irradiance, xcex5 is the emittance of the spacecraft surface, A2 is the emittance area, S is the Stefan-Boltzman constant, and T is temperature (absolute).
Additionally, spacecraft surfaces are exposed to space plasma environments that may create a surface electrostatic charge build-up. The charge build-up, if not controlled, may reach levels where discharges will occur that may damage spacecraft components or structure or may create deleterious electromagnetic interference of electronic components and circuitry, rendering a satellite system inoperative. Thus, satellites also benefit from surfaces that have acceptable electrostatic discharge control properties so that the surfaces may be reasonably grounded to prevent surface charge build-up.
Thermophysical properties of a material are measured in terms of the ratio of solar absorptance to hemispherical emittance (xcex1/xcex5). Solar absorptance is a measure of the percentage of solar energy that is absorbed by the material. Hemispherical emittance is a measure of the emittance of heat from a surface in all directions. Emittance is the heat exchange of a surface with the surface""s surrounding environment at a given temperature. Typically, emittance is given as a ratio of the heat exchange of a real surface to the heat exchange of a perfect absorber at a given temperature. Thus, the ratio of solar absorptance to hemispherical emittance that defines the thermophysical properties of a coating or material is a ratio of how much heat is absorbed by the material to how much heat is emitted by the material. Consequently, as indicated by Equation (2), a low ratio (xcex1/xcex5 less than 0.5) is preferred to obtain lower temperatures to protect underlying components from damage from solar heating.
Additionally, other current coatings may provide electrostatic discharge protection for a surface. Electrostatic discharge protection may be measured in terms of surface resistivity, also known as surface resistance. Surface resistivity (xcfx81) is the resistance of a material to the flow of electrical current over the surface of a material. Consequently, a low resistivity (106 to 108 Ohms/square) is preferred in order to prevent electrostatic charge from collecting or xe2x80x9cbuilding upxe2x80x9d on a surface.
As may be apparent to those of skill in the art, a surface coating that exhibits both good thermal protection and good electrostatic discharge protection is highly desirable for use in space applications. Currently, protective coatings for space system surfaces and/or electrical circuit surfaces may be produced to provide good thermal protection for the surface, or coatings may be produced to provide good electrostatic discharge protection for the surface. That is, current coatings do not possess an acceptable balance of thermophysical and surface resistance properties, or they may not be applied to flexible membrane surfaces because of their brittle nature. Current techniques of overcoating materials for thermal protection also upset the desired balance of thermal control and surface resistance. Thus, there is a need for a protective coating that both protects a surface from over-heating and protects a surface from electrostatic charge build-up and that may easily be applied to both flexible and rigid surfaces. Additionally, there is a need for a production system for depositing such a coating on a surface.
Current coating systems, such as paints, are thick (txcx9c5 mils or 1.25xc3x97106 Angstroms) and may not be used on flexible membranes or require conductive pigments to obtain an acceptable combination of thermophysical (xcex1/xcex5 less than 0.5) and surface resistance (xcfx81 of 106 to 108 Ohms/square) properties. Overcoatings of transparent conductive oxides are also used to obtain lower surface resistance properties. But such oxide overcoatings affect the thermophysical properties and are brittle and thus not conducive for flexible membrane applications. Single semiconductor materials such as silicon or germanium may provide acceptable thermophysical coatings but have unacceptably high surface resistance (xcfx81 greater than 1014 Ohms/square). Conductor materials such as platinum, silver, gold, copper, or stainless steel have acceptably low surface resistance properties, but have unacceptably high thermophysical properties (xcex1/xcex5 greater than 1). Therefore, it is highly desirable to have a thin coating with low thermophysical properties for heat control and low surface resistance for electrostatic discharge control. It is especially desirable for such a coating to be flexible and adhere to surfaces.
Currently, the preferred method for producing and applying a very thin (t less than 2000 Angstroms) coating on membrane or rigid structural surfaces is through sputtering deposition. Sputter deposition, or sputtering, refers to the process of bombarding a solid (referred to as the target) with high energy ions from a plasma which causes atoms or molecules to be ejected from the target and re-deposited on the surface of the substrate located near the target. Thus, the atoms from the target form a controlled, uniformly thin film on the substrate.
Current attempts to create a sputtering target with a balance of thermophysical and surface resistance properties have involved processes of physically mixing or combining materials into alloys to form a sputtering target. These processes are time consuming because the different components must be integrated into a single alloy. These processes are also expensive due to the complexity of the alloy process. Additionally, it is difficult to arrive at the precise balance of materials that will produce the desired thermophysical properties and surface resistance, and the target alloy material may not be altered once it is cast. Furthermore, alloys produced may generate unwanted heat during sputter deposition and may react unfavorably to the conditions of sputter deposition. Thus, there is a need for a cost-effective method of easily creating a sputtering target that will produce the desired thermophysical and surface resistance properties in a protective coating.
The preferred embodiments of the present invention relate to a system and method for depositing a protective coating on a surface. That is, the preferred embodiments of the present invention relate to a system and method for depositing a very thin coating with desired thermophysical and surface resistance properties on a surface. The system includes a coating device including a first material for coating, a second material for coating and a surface to be coated. Preferably, the coating device is a sputter deposition device. Preferably, one of the first and second materials is a semiconductor material, such as silicon or germanium, for example, and the other is a conducting material, such as stainless steel, silver, aluminum, gold, and/or platinum, for example. The first material and second material have differing thermophysical properties and surface resistance properties. In a preferred embodiment, the first material and the second material are deposited on the surface in a predetermined proportion to yield an intermixed coating having desired thermophysical and surface resistance properties. The proportion may be controlled by controlling exposed surface area of the first material and exposed surface area of the second material, as well as a magnetic field applied to the first and second materials.
The method includes simultaneously depositing at least a portion of the first material and at least a portion of the second material on a surface to form a thin intermixed coating. In a preferred embodiment, the first material is a semiconductor or insulating material and the second material is a conducting material. Preferably, the materials are deposited on the surface in a predetermined proportion to yield a coating having desired thermophysical properties and surface resistivity.